In synchronous telecommunication networks of the above-mentioned kind, the synchronization of the individual exchanges is usually done according to the hierarchical master-slave principle through digital signal connections, as shown, for example, in FIG. 1. In this case, an exchange (e.g., V2) can be both a master exchange (transmitter of a reference frequency signal) with respect to one or more exchanges of the same rank (e.g., V3) or of a rank subordinated to it by a hierarchical level HL (e.g., V4), and a slave exchange (receiver of a reference frequency signal) with respect to one or more exchanges of the same rank or of a rank that is superior by a hierarchical level HL (e.g., V1).
If in an exchange (e.g., V1) the highly accurate pulse signal (exchange pulse signal) generated by its highly stable central clock pulse generator should fail the clock generators in this exchanges downstream of the central clock pulse generator will no longer be supplied with this exchange pulse signal, whereupon, due to their comparatively low internal stability, the accuracy of their frequency will drop abruptly for practical purposes.
Since the reference frequency signals (f.sub.R1, f.sub.R2) are generated by these group clock pulse generators for exchanges (e.g., V2) of the same rank or of a rank subordinated by a hierarchical level, the accuracy of the reference frequency signal (f.sub.R1) received by such an exchange will likewise drop abruptly. In order to be able to detect this loss of accuracy, the central clock pulse generators of exchanges possess frequency monitors. These frequency monitors monitor the deviation between the frequency of the received reference frequency input signal (e.g. f.sub.R1) and the frequency of the output signal of one of the two central clock pulse generators present in an exchange (e.g. V2) and, when a specific frequency deviation--hereinafter simply called a threshold--is exceeded, trigger a frequency alarm.
Since the frequency deviation cannot be measured directly, the exceeding of the frequency deviation threshold is detected by measuring the drift of the corresponding phase deviation between the received reference frequency input signal and the generated reference frequency output signal within a certain measurement period (measurement cycle).
If the reference frequency input signal and the reference frequency output signal are independent of one another, then a drift in the reference frequency will sooner or later produce, quickly or slowly, a certain frequency deviation and thus a frequency alarm. However, if the reference frequency output signal is synchronized to the reference frequency input signal, then it will follow the reference frequency input signal in accordance with the time constants of the phase control loop responsible for the synchronization in the central clock pulse generator (dependent output frequency).
Whether a frequency alarm occurs in the case of a dependent output frequency depends on how large the frequency drift and the time constants of the control loop are. If the phase control loop is in, for instance, an operating mode in which its time constants are very large, the frequency of the reference frequency output signal cannot follow the frequency of the reference frequency input signal fast enough when there is a rapid frequency drift. The frequency deviation between the frequencies of the reference frequency input and output signals will finally be so large that a frequency alarm will be triggered.
On the other hand, when the frequency drift is sufficiently slow, the frequency of the reference frequency output signal will follow the frequency of the reference frequency input signal, so that the frequency deviation between the two never reaches the threshold for the frequency alarm.
We shall now refer again, as an example, to the exchange V2 and assume that, due to a rapid frequency drift in the frequency of the reference frequency input signal f.sub.R1 at that moment, a frequency alarm is triggered.
According to Telcom Report 1986, Vol. 4, pages 263 through 269, a check is then made to determine whether another unaffected reference frequency input signal, namely f.sub.R0, is present. If this is the case, the system switches to this reference frequency input signal.
If no other reference frequency input signal is present, the control loop of the central clock pulse generator of the exchange V2 switches to the hold-over mode, in which its voltage-controlled oscillator (VCO) is kept at the last correct setting.
In the hold-over mode, the accuracy of the highly stable central clock pulse generator of the exchange V2 drops--through aging, for example, of the highly stable voltage-controlled oscillator--only very slowly, so that this drop cannot be detected by the frequency monitors of the downstream slave exchanges (see, e.g., V4, V5 and V6) according to the above explanation, if the reference frequency output signal of those downstream slave exchanges is dependent on the reference frequency output signal from exchange V2 (i.e. there are no frequency alarms in the downstream exchanges).
Therefore, the phase control loops of the central clock pulse generators of these slave exchanges remain in the synchronous mode, in which the phase control loops synchronize with large time constants to the reference frequency input signals of the moment. The result is that the accuracy of the network N formed by these slave exchanges drop synchronously with the frequency of the reference frequency output signal f.sub.R2 of the exchange V2.
If the master exchange V2 remains sufficiently long in the hold-over mode, the frequency deviation of the network N from the original reference frequency f.sub.R2 of the master exchange V2 when it switched to the hold-over mode will reach or exceed the threshold value for the frequency alarm at a certain point in time, without triggering a frequency alarm at an exchange of the network N. If, at a later time, the master exchange V2 is resynchronized by an again intact reference frequency e.g. f.sub.R1, then exchange V2 enters a start-up mode, in which the time constants of the phase control loop are as small as possible, in order to bring the frequency of the reference frequency output signal f.sub.R2 as quickly as possible to the frequency of the reference frequency input signal f.sub.R1.
Because of the high rate of change of the frequency of the reference frequency output signal f.sub.R2 and the large time constants of the phase control loops of the slave exchanges in the network N, frequency alarms will occur in these slave exchanges causing those exchanges to switch over to the hold-over mode. Thus, the desired resynchronization of the network N through resynchronization of the master exchange V2 will not take place.
The reason why exchange V2 remains so long in the hold-over mode may be attributed, on the one hand, to the disappearance of all its reference frequencies f.sub.R0, f.sub.R1 for a long time, or, on the other, to the fact that it is being operated in the master mode, i.e., as an independent frequency standard for the network N, but, has not, in this case, been attended to--that is, resynchronized--for too long a time.